Signals representative of images captured by a digital sensor must be processed and converted to the JPEG (acronym of “Joint Photographic Experts Group”) format to be capable of being viewed. Such a conversion involves a loss of information contained in the signal representative of the image captured at the sensor output. The quality of the image is also reduced due to passing from a format coded over a number greater than eight bits, such as twelve or fourteen bits, for example, to the JPEG format coded over eight bits.
There are image capture means capturing at least two signals representative of images in a RAW format then each signal representative of an image undergoes an image processing independently of the others. Then the images are combined, and the combination of images is converted to the JPEG format.
Capture means for capturing at least two signals representative of images allow a conversion of each signal representative of an image which undergoes image processing, then each processed image is converted into a format configured for being displayed on a screen, such as the JPEG format or the TIFF (acronym of “Tagged Image File Format”) format. This conversion involves a loss of information. Once the conversion is performed, the images are stored and combined into an image in the JPEG format. There is therefore a double loss of information and quality, the first at the time of converting the images and the second at the time of combining the images. This loss of information limits the possible post-processing.
Other image capture devices capture at least two signals representative of images in a RAW format. The signals representative of images may undergo TNR (acronym of “Temporal Noise Reduction”) processing or HDR (acronym of “High Dynamic Range”) processing, for example. The images are combined during this processing. The combination causes a loss of information, notably the quantity of combined pixels and the way in which the combination has been performed. The combined image is then processed by means of an ISP (acronym of “Image Signal Processor”) then stored. The combined image may undergo post-processing by means of specialized software. However, this loss of information limits the possible post-processing. For example, a noise reduction post-processing algorithm does not have access to noise characteristics that depend for each pixel on the quantity of combined pixels and the way in which the combination has been performed.
A TNR algorithm is notably disclosed in the publication “A High-Quality Video Denoising Algorithm based on Reliable Motion Estimation” by C. Lieu and W. Freeman. The disclosed TNR denoising algorithm is an algorithm for denoising by patch by applying a method of the k nearest neighbors which seeks similar patches in a neighborhood the size of which may be the size of an image.
The processing of a set of signals representative of images is a process the complexity of which is far superior to the processing of a single signal representative of an image. The processing of a plurality of signals representative of images may multiply the complexity of the algorithms by the number of processed images. The computing power of digital movie cameras and digital still cameras is limited is the algorithms generally used with a single image are not executable in a reasonable time, unless image quality is sacrificed by using a less complex algorithm.
In the case of HDR processing, fusing a set of signals representative of images into a single image makes it possible to create an image the dynamic of which is far superior to the dynamic of each initial image. This dynamic is often not representable in the algorithms embedded in the ISPs integrated in digital movie cameras and still cameras. The image is generally quantized for reducing the dynamic of the image and allowing its processing by the ISP.
An HDR algorithm is notably disclosed in the publication “Exposure Fusion” by T. Mertens et al. The HDR algorithm fuses a sequence of images the exposure of which varies between predetermined limit values for obtaining a high quality image without converting prior use of another HDR algorithm.
The publication “All Smiles: Automatic Photo Enhancement by Facial Expression Analysis” by R. Shah and V. Kwatra discloses a system of automatic enhancement of photographs by facial expression analysis in different images. The algorithm assigns a score to each image, and in the image with the highest score faces with low scores are replaced by faces with high scores included in other images.
Patent application U.S. Pat. No. 3,398,634 A discloses a method for having a clear image by stacking clear parts of images. “Focus stacking” algorithms are also known in the prior art.
The publication “Automatic Generation of Action Sequence Images from Burst Shots” by S. Chen, et al. discloses a fusion algorithm for fusing images in which objects are in different locations between each image. The algorithm makes it possible to display the objects in different locations in the fused image or to remove the objects from the fused image.
There is also post-processing software on computer, notably the Adobe LIGHTROOM® (registered trademark) software package, which takes a plurality of images at the input in a RAW format. The user may then manually control an HDR processing on the images. The software is incapable of automatically selecting a processing to be applied, such as a TNR or HDR algorithm, for example.
There are also digital cameras marketed by LYTRO® (registered trademark), making it possible to refocus an image after the image has been captured. The device a single image produced by a special optic.
The Digital Camera Utility (registered trademark) software makes it possible to compose an image from a plurality of signals representative of images in a RAW format, through an HDR algorithm. This software is incapable of automatically selecting a processing to be applied, such as a TNR or HDR algorithm, for example.
Finally, the DNG (acronym of “Digital Negative”) format makes it possible to store a plurality of signals representative of images, such as the signal representative of an original image in a RAW format of the manufacturer and the RAW format converted into DNG.
The DNG format includes a signal representative of a digital image in a standardized format and a set of capture metadata of the signal in a standardized format. The DNG format comprises free fields in which signals representative of additional images or metadata may be recorded. The specification of the DNG format states that the DNG image format is based on the TIFF image format. The DNG format is compatible with the TIFF format. The TIFF format makes it possible to store images supplementary to a main image. However, the TIFF format has a limited size which limits the number of supplementary images that can be stored in a file in the TIFF format. The supplementary images are stored by means of an EXIF (acronym of “Exchangeable Image File Format”) subIFD (acronym of “Image File Directory”) metadata tag.
The aforementioned devices involve a loss of information on the combination of signals representative of images and a decrease in the quality of the image. The aforementioned devices are unsuitable for an automatic application of suitable high-quality processing, such as TNR or HDR processing, for example, to the captured images and to the capture mode of these images.